Writing automated tests is a tried and tested way to improve the quality of software. In the initial phase of development, tests help to verify that the code functions correctly. In Test Driven Development, tests are written before the code, so any knowledge about the ins and outs of the implementation won't influence the writing of the tests. After the initial development, as changes are made over time, a comprehensive test suite can quickly point out unintended changes in the behavior of the code, so the bugs can be fixed before the software is shipped. Importantly, the tests are automated, so they are very cheap to run in terms of time.

What do we want from a test suite?

There are a range of properties that are good to have in a test suite.

• It often allows bugs to be pinpointed to a relatively small area of the code base, saving debugging time...